It's not an option to vote for but my favourite, by far, is Wildlife on One. All 253 episodes were narrated by David Attenborough, so I wouldn't hesitate to call it an Attenborough series.
I think a big part of why I love it so much is because of its fairly short runtime per episode (each is only half-an-hour long). That means an episode can focus on a species that may get overlooked were only a full hour slot available. I cannot imagine, for example, there being hour-long episodes about springhares (Kalahari Bigfoot, 1989), frigatebirds (Birds Behaving Badly, 1999), peacocks (The Tale of the Peacock and the Tiger, 1995), nautilus (Nautilus: 500 Million Years Under the Sea, 1987), the Mona ground iguana (An Island Shall a Monster Make, 1980), brown hares (Shadow of the Hare, 1993), red-tailed hawks (Redtail, 1985), Thomson's gazelles (A Graze with Danger, 1994), white-fronted bee-eaters (The Bee Team, 1988), jellyfish (The Swarm, 1993), Southern ground hornbills (Thunderbirds, 1996) or bulldog ants (Encounter Underground, 1982). And that is just a fraction of the interesting animals that got their own episode throughout the show's runtime.
If I had a magic wand that could bring back one former wildlife documentary series, it would be this one.